Patients with OHS are efficiently managed with long term home-based nocturnal noninvasive positive pressure ventilation (NIPPV). Several NIPPV devices offer the feature of automatically adjusting pressure support (and/or respiratory back-up rate) on the basis of a pre-determined "ideal tidal volume" or "ideal ventilation". However algorithms used to achieve these ideal targets are different among different commercialized devices and the relative efficacy from an algorithm compared to another remains unknown. The main objective is this study is to compare two commercialized NIPPV that have this option